Process for increasing the resistance of polyacrylonitrile fibers to discoloration



United States Patent C) PROCESS FOR INCREASING THE RESISTANCE OF POLYACRYLONITRILE FIBERS T DISCOLOR- ATION Vera Isabella Furness, Coventry, England, and Hector Douglas Mackenzie, Mhlambanyati, near Mbabane, Swaziland, assignors to Courtaulds Limited, London, England, a British company No Drawing. Filed Apr. 28, 1961, Ser. No. 106,181

Claims priority, application Great Britain May 6, 1960 4 Claims. (Cl. 117-1383) This invention concerns the treating of artificial fibres of polyacrylonitrile to render them more stable to discoloration. In the word fibres we include continuous filaments as well as staple fibre. By polyacrylonitrile we mean homopolymers of acrylonitrile and copolymers containing at least 80 percent by weight of acrylonitrile.

Polyacrylonitrile fibres exhibit a tendency to discolour which may display itself when the fibres are subjected to heating, for example during drying or hot-stretching of the fibres, or during steaming. Various suggestions have been made for modifying the polyacrylonitn'le solution which is spun to form the fibres, in order to improve the colour of the freshly-spun fibres. However, it is desirable that a treatment he made available, which can be applied to the fibres after spinning, for improving the colour stability of spun fibres of poor stability. It is an object of this invention to provide such a treatment.

According to this invention, a process for treating artificial fibres of polyacrylonitrile to render them more resistant to heat-discoloration and/ or steam discoloration comprises applying to the surface of the fibres, before they have been first dried, an aqueous solution or suspension of a reducing agent which does not have any adverse efiect on the fibres. Typical reducing agents which may he used are thiourea dioxide (also known as formamidine sulphinic acid), sodium and potassium metabisulphite, formaldehyde and addition compounds of formaldehyde with compounds which do not themselves adversely affect the fibres. The formaldehyde addition compounds which may be used include that with sodium bisulphite, while sodium formaldehyde sul-phoxylate is unsuitable because of the deleterious effect of the second component. Formaldehyde may also be used in the form of its polymer paraformaldehyde.

The application of the solution or suspension of reducing agent to the fibres may advantageously be carried out by passing a continuously moving bundle of filaments through a bath of the agent at a stage in the fibre production-line which precedes the drier. If desired, the treatment according to this invention may be combined with other treatments such as the application of a finish to the fibres.

The solution or suspension of reducing agent may contain between 0.1 percent and 10 percent of reducing agent by weight, preferably less than 5 percent. Solutions containing of the order of 0.5 to 2. percent of reducing agent by weight have proved particularly useful.

The treatment according to this invention may be carried out using a solution or suspension which is at room temperature or at an elevated temperature. If the treatment is combined with any other treatment of the fibres which makes elevated temperatures necessary or desirable, such temperatures may be used without objection.

The invention will now be further described by means of the [following examples, in which percentages are by weight unless stated otherwise:

Example 1 Undried polyacrylonitrile filaments spun from a solution in sodium thiocyanate solution were immersed for "ice 2 seconds at 20 C. in a solution of sodium formaldehyde bisulphite prepared from 6 g. of a 40 percent solution of formaldehyde, 6 g. of sodium metabisulphite and 350 cc. of water. They were then dried at C.

Treated and untreated fibres were compared (a) after steaming for half an hour under a steam pressure of 20 psi. and '(b) after heating in air at 150 C. for 30 minutes. In each case the treated fibre showed less discoloration than the untreated fibre, to an extent readily detectable by the unaided eye.

In the remaining examples, the fibre colour where given has been determined as fibre yellowness (Y) from the equation RB G where R, B and G are the reflectance ratios in red, blue and green respectively, as measured on a Colormaster diiferential colorimeter.

Example 2 Undried polyacrylonitrile filaments were treated by immersion for 5 seconds in a bath consisting of 5 g. of paraformal dehyde suspended in cc. of Water. The fibres were then dried at 80 C.

After steaming for half an hour at 20 psi. the treated fibres showed a colour of 0.082, compared with 0.180 for steamed untreated fibres.

Example 4 Undried polyacrylonitrile filaments were immersed for 30 seconds at room temperature in filtered aqueous solutions of thiourea dioxide at pH 5.4, after which excess liquor was squeezed out by rollers and the fibres were dried for 2 hours at 80 C. Untreated fibres and fibres treated in solutions of varying concentration were subjected either to heating at C. for 30 minutes or to steaming for 30 minutes at 10 p.s.i. The following table gives the fibre colour of the various samples:

Colour Colour Colour Treatment Bath after after after drying heating at steaming 180 C.

Untreated 0. 094 0. 250 0. 1% Solution 0. 101 0. 104 2% solution 0.097 0.126 0.107 5% solution 0. 083 0.106

It will be seen from the foregoing Examples 1 to 4 that treatment according to the invention has the efiect of improving the colour of the fibres after heating to higher temperatures or steaming, and in some cases improves the fibre colour of the dried fibre.

What we claim is:

1. A process for increasing the resistance of polyacrylonitrile fibres to discoloration, comprising applying to the never-dried fibres in an aqueous medium a reducing agent which has no adverse effect on the fibre.

2. A process for increasing the resistance of polyacrylonitrile fibres to discoloration, comprising immersing the never-dried fibres in an aqueous bath containing compounds with formaldehyde of a compound which 0.5 percent to 2 percent of sodium formaldehyde bisulhas no adverse effect on the fibres. phite. 4. A process according to claim 3, in which the aqueous A Process fOT increasing the resistance of P 3 medium contains 0.1 to 10 percent of the reducing agent. acrylonitrile fibres to discoloration, comprising applying 5 to the never-dried fibres, in an aqueous medium, a re- References Cited in the file of this patent ducing agent selected from the group consisting of sodi- UNITED STATES PATENTS um metabisulphite, potassium metabisulphite, thiourea dioxide, formaldehyde, parafonmaldehyde and addition 2,768,995 Mosse 1956 

1. A PROCESS FOR INCREASING THE RESISTANCE OF POLYACRYLONITRILE FIBRES TO DISCOLORATION, COMPRISING APPLYING TO THE NEVER-DRIED FIBRES IN AN AQUEOUS MEDIUM A REDUCING AGENT WHICH HAS NO ADVERSE EFFECT ON THE FIBRE. 